Various types of vehicle behavior controller are known in the art. One such controller, which individually controls the hydraulic pressure of a wheel cylinder (braking force) of each wheel to prevent spin or drift out, etc., while the vehicle is turning or steering, is disclosed in JPA 8-310366.
This controller detects a steering amount, wheel speed, yaw rate, and tire grip limit. First, a target yaw rate in the grip limit is determined from these parameters, and it is determined whether or not the vehicle is in an oversteer or understeer turning state wherein the grip limit of the tire is exceeded.
When the vehicle is in a turning state where the tire grip limit is exceeded, the hydraulic pressure of the wheel cylinder of each wheel is controlled so that an actual yaw rate approaches a target yaw rate. In this way, the vehicle is run within such limits that the unnatural behaviour which can occur during turning is prevented.
In another type of controller, such as a wheel anti-skid controller, the hydraulic pressure of the wheel cylinder (braking force) is controlled so that a wheel slip ratio does not exceed a predetermined value.
This usually estimates the vehicle speed (running speed) from the plural wheel speeds, and computes the slip ratio of the wheel by comparing the wheel speed and vehicle speed, and controls the hydraulic pressure of the wheel cylinder so that this slip ratio is an ideal slip ratio whereby the frictional coefficient of the wheel on the road is maximized.
However, in a vehicle where a vehicle behavior controller and a wheel slip controller are both installed, these functions may mutually conflict.
For instance, if a large braking force is applied by behaviour control to prevent vehicle spin, the wheels may lock. The wheel slip controller then functions to decrease braking force to release the wheel lock.
However, as the behavior of the vehicle is unstable, behavior control again applies a large braking force, and when the wheels lock again, braking force is again decreased by slip control.
These operations are performed repeatedly, giving rise to "hunting" and making the total control unstable.